In recent years, from the environmental point of view, there are increasing demands for automobiles to replace conventional gasoline-engine cars. In particular, hybrid cars that combine a gasoline engine with a motor to offer higher fuel efficiency, lower noise, and the like are receiving interest. Motors used in such hybrid cars are required to have extremely high output, and also, they are used in an extremely severe environment for a long period of time. Accordingly, an insulating film used in the motor is required to have high heat resistance and high hydrolysis resistance.
With the recent advent of electrical appliances and electronics with reduced size and increased performance, improvement of heat resistance, amount of oligomer extracted relative to refrigerant, hydrolysis resistance, and like characteristics of insulating films for conventional motor usage, such as those for use in motors for compressors of refrigerators, air conditioners, and the like, has been required, and various studies have been made thereon. For example, as a result of modification of heat resistance (heat-resistance class E) of a usual polyethylene terephthalate film (hereinafter referred to as PET film), a PET film with heat-resistance class B (class B PET film) has been used. Further, according to JP-A-2001-76536 (Patent Document 1), JP-A-2002-273844 (Patent Document 2), etc., a polyethylene naphthalate film (hereinafter sometimes referred to PEN film) and the like have also been in use.
A PEN film, in particular, is an insulating film of high heat-resistance class, and thus is often used by itself. Accordingly, in many cases, the film is used as a thick film having a thickness of not less than 200 μm. When such a thick PEN film is processed into an insulating material for an electrically insulating member called slot or wedge, delamination, a phenomenon that stacking (stacked structure) of molecules undergoes separation between the planes of the aromatic rings, often takes place at the time of punching or bending, accompanied by the problem of low yield.
As a method to suppress delamination during processing of a PEN film, JP-A-6-335960 (Patent Document 3), for example, has proposed a method in which the temperature for PEN film processing, the punch curvature, and the like are devised to reduce the frequency of delamination. However, employment of such a method requires improvement of the processing apparatus. In addition, in winter when atmospheric temperature and humidity are low, the effect of the method is occasionally insufficient.
Meanwhile, when a PET or PEN film conventionally used as insulating film is used as an insulating film for automobile driving motors, the performance thereof has been insufficient in terms of hydrolysis resistance.
Consequently, in order to use a PEN film as an insulating film of an automobile driving motor, there now is a demand for a biaxially oriented polyester film for automobile driving motors, having high-temperature dimensional stability, and which also has higher hydrolysis resistance than a conventional PEN film and exhibits excellent delamination characteristics.    Patent Document 1: JP-A-2001-76536    Patent Document 2: JP-A-2002-273844    Patent Document 3: JP-A-6-335960